It is well known in the art of drilling, completion and workover of earth boreholes in the oil, gas and geothermal industries to run strings of oilfield tubulars into and out of such boreholes, sometimes referred to as “tripping in” or “tripping out”. Such tubulars can be, for example, drill pipe, drill collars, casing and tubing. It is also well known to use elevators in such tripping in or out operations to lift or lower such tubulars out of, or into the wells. The handling gear for such tubulars is oftentimes much alike in principle for all sizes but the difference in scale is impressive. Well casing with a diameter of six feet, with a two inch wall thickness, is not uncommon.
Elevators in the prior art typically are hinged, heavy clamps attached to a hook and traveling block by bail-like arms, sometimes referred to simply as “bails”. Such elevators oftentimes use one or more doors which are themselves quite heavy, and which may require two or three strong men to close or hinge the one or two doors around the tubular. Doors are a common feature but there are single door and split door types. One type simply hinges to open to admit or eject pipe. In hoisting a joint of drill pipe, the elevators are latched onto the pipe just below the tool joint (coupling) which prevents the drill pipe from slipping through the elevators. Similarly, in lifting casing or tubing, the sections of such tubulars have either an upset end, i.e., one in which the O.D. is larger than the primary diameter of the casing or tubing, or they are joined together with a collar having an enlarged O.D. In all of these type of operations, the elevator when hinged to the closed position, i.e., when the one or two doors are closed shut, the internal diameter of the elevator is less than the O.D. of the end of the enlarged tool joint, upset, or collar to prevent the tubular from slipping through the elevator.
Handling practices differ between small and rather large diameter pipe. Each section of very large pipe will typically be picked up from the horizontal position and swung to the vertical for stabbing into the connection of the assembled tubular string. Such large pipe, for example, large diameter steel casing, presents special problems. When elevators are placed on the horizontal pipe they have to pivot to orient the elevator throat opening downward. That leaves the doors, on door-type elevators, swinging on hinges. The doors on a large elevator may weigh several hundred pounds. To close such doors, drilling crew men place themselves in hazardous situations. The rigging devised to get the doors closed often is creative, but risky.
An elevator with doors needs clearance for the doors to swing in the closing arc under the pipe being engaged. The pipe has to be elevated, or clearance otherwise provided, for the swinging door.
The elevators discussed above are of the so-called “non-slip” variety. There are other elevators which grasp the tubular and can be used to then hoist or lower the tubular, but the grasping elevators are typically used with the light weight tubulars.
The elevators of the “non-slip” variety have generally been constructed with doors (generally, one or two) which open to allow the insertion or removal of the tubulars. These doors have traditionally been heavy, slow in operation, difficult to handle and present a considerable safety hazard to the operator. Also, the balance point of the elevator will change dramatically when the doors are opened, thus exacerbating handling problems and adding danger to the operator.
Especially with very heavy tubulars, for example, 20″–30″ casing, the tubular is initially in a horizontal position, laying in place, for example, on or near the floor beneath a derrick, and the hinged door elevator is lowered near the point of attachment to the tubular. The derrick hands then are required to open the very heavy door or doors, which may weigh several hundred pounds, to allow the elevator to be placed over the tubular. Moreover, because the door or doors must close around the tubular, the tubular end around which the elevator is placed must be above the derrick floor.